Process and bath for descaling metals



Unit-d. States Patent 2,796,366 PROCESS AND BATH FOR DESCALING METALSGiles F. Carter, Niagara Falls, N. Y., assignor to .E. I. du Pont deNemours and Company, Wilmington, Del., a corporation of Delaware NoDrawing. Application October 31, 1955,

Serial No. 544,059

Claims. (c1. 134-2 This invention relates to a fused bath and to aprocess for descaling metals, ferrous metals in particular.

Gilbert, in U. S. Patent 2,377,876, shows a method for reducing oxidescale on metallic surfaces. In the patented method the metal to bedescaled, usually iron or one of its alloys, is dipped into a moltenbath of sodium hydroxide containing up to about 20% by weight of sodiumhydride. For a decade or more this process has been widely used in themetal trades.

The utility of the Gilbert bath has largely been confined to thedescaling of stainless steel. In particular it has been impracticable todescale low-grade carbon steels therein because the reduced scaleadheres tightly to the surface of such metal. Removal of such reducedscale is too expensive for commercial acceptance An object of thisinvention is, consequently, to effect improvements in the process fordescaling metals in hydride-containing fused baths.

A specific object is provision of a method for descaling carbon steelsin hydride-containing fused baths.

Another object is provision of an improved fused bath for descalingmetals.

A further object is provision of a fused bath for descaling carbonsteels.

The above-mentioned and yet other objects are achieved in accordancewith this invention by a process in which a metallic workpiece,generally formed of carbon steel, is first immersed in a molten bath ofalkali metal hydroxide containing iron in addition to alkali metalhydride. This step reduces the scale to a form loosely held to thesurface of the workpiece. The workpiece is then quenched in water whilestill hot and the loose scale removed therefrom.

The fused bath which serves as the basis of the invention is mostconveniently prepared by forming a descaling bath of the type describedby Gilbert, i. e. a molten alkali metal hydroxide containing up to 20%by weight of an alkali metal hydride, and then adding thereto a mixtureof an iron oxide and an alkali metal oxide. The bath is subsequentlymaintained by analyses followed by addition of needed components.

The preferred alkali metal hydroxide for use in the invention is sodiumhydroxide. Between about 50 and 95% by weight of this compound isnormally utilized. The hydroxides of other alkali metals may besubstituted in whole or in part for that of sodium but to no particularadvantage. For economic reasons, up to about 50% by weight of thecaustic may also be replaced by sodium carbonate, sodium chloride ormixtures thereof or by other inert alkali metal salts.

In like manner sodium hydride is the preferred hydride for addition tothe bath. Between about 0.3 and 5% by weight of the bath should consistof this material. Larger percentages up to about 20% are usable butunnecessary. The hydride can be added preformed to the bath or generatedin a separate compartment of the bath itself. Generation is easilyaccomplished by adding sodium metal to molten caustic and passinghydrogen or cracked ammonia therethrough. Potassium or other alkalimetal hydrides can be substituted for sodium hydride if it is sodesired.

To obtain reduced scale easily removable from the ICC workpiece, thebath should contain about 02-10% by weight of dissolved iron, calculatedas the metal. About 0.3-1% by weight is preferred. .The best operationof the bath occurs when some particular weight of iron Within the rangeis chosen and the Weight is maintained constant. The iron is preferablysupplied to the bath, not as the metal, but as an oxidic compound suchas FeO, FezOa, F6304, NaFeOz or the like. Iron oxide scale and ores canalso be used.

Iron is soluble to a maximum extent of about 0.1% by weight inconventional hydride baths. Some method of increasing the solubility istherefore necessary. Addition of an alkali metal oxide, preferablysodium oxide, along with the iron will accomplish this result. Thequantity of sodium oxide utilized can vary greatly from 0 to about 3times the weight of iron oxide (FezOa). The preferred weight is l-2times that of FezOa. A mixture of sodium ferrite (NaFeOz) and NazO canreadily supply these requirements. Expressed in terms of bath weight,sodium oxide may constitute between about 0.3 and 10% thereof, thelarger quantities of course being employed with the larger amounts ofiron oxide. Sodium oxide can also be supplied to the bath in other waysthan by direct addition. Direct generation by the reaction betweenmetallic sodium and caustic, for example, generally occurs when the bathis in operation.

There is no direct way of determining sodium oxide in hydride-causticbaths. The form in which it is present or even its presence-isconsequently unknown. Equilibrium considerations suggest, however, thatappreciable quantities of NazO are dissolved as such in the bath, atleast as long as the partial pressure of hydrogen is not too high. Ironmay begin to precipitate if this partial pressure rises above about 0.5atmosphere. The effect on the solubility of iron suggests that some maybe tied up in compounds of the type NazO-xFeO, where x is probably 1.

Iron, of course, does not exist in the bath as the metal. Its exactform, like that of the sodium oxide, is unknown although the presence offerritesseems likely. Direct and conventional analyses for iron are,nevertheless, possible and the concentration can be reported in terms ofany compound. While in this specification weights are reported in termsof metallic iron. it will be appreciated that an iron compound isactually dissolved.

A nonoxidizing atmosphere may be employed to protect the present bathsfrom attack by air and oxygen. This can be provided by nitrogen, argon,neon or other inert gas. Most conveniently, however, the bath may beprotected by the use of floating covers of a form of carbon such asgraphite or-charcoal. Floating pans or lids fastened to the containerfor the bath may also be used.

The mechanical operation of the descaling baths of this invention isquite simple and substantially that of the prior art. The workpiece tobe descaled is given a preliminary brushing, if desired, and dipped forabout /230 minutes in the molten bath. The exact time required will varyto some extent with the thickness of the scale. The temperature of thebath should be between about 325 and 550 C. Around 500 C. issatisfactory and preferred. After the immersion, the workpiece isremoved and quenched while hot in water. Further treatments such aswashing in a high-pressure Water spray, scrubbing or rinsing in diluteacid may be carried out if desired. The quench, however, removessubstantially all of the finely divided reduced scale from the metal.

Maintenance of the bath during prolonged operation involves principallythe addition of sodium hydroxide, sodium and iron. Caustic may be addeddirectly to the :bath. Sodium hydride may also be added as such but ispreferably generated in situ by the reaction of hydrogen gas on sodium,the method of generation conventionally employed in hydride descalingoperations. The ratio of hydrogen to sodium metal should, however, becarefully controlled during this generation to avoid precipitation ofiron. Usually this ratio will vary between Mr and 2 liters of hydrogenfor each gram of sodium. If the ratio is less than that recommended, thebath is likely to become corrosive. An increase in the HzzNa ratiowithin the range given will decrease the solubility of iron while adecrease in the ratio will have the reverse effect.

Iron should be added along with sodium oxide as the need therefor isindicated by analysis. Great care should be exercised if sodium oxide isadded alone since a bath containing the oxide and no iron will corrodethe container or any other ferrous metal it contacts. This fact explainsthe desirability of keeping the weight of iron constant.

It will be understood that the iron-containing baths of this inventionare useful for descaling bases other than carbon steel. In general, ironbases or alloys carrying oxide scale can be descaled therein. Variousnonferrous metals such as copper and nickel and alloys such as nichromecan also be cleaned by these baths. In fact, almost any metal that canwithstand the high alkalinity of the molten caustic can be treated inthe iron-containing baths.

There follow some examples which illustrate the inven tion in moredetail. In these examples all percentages are weight percentages. Totalgas pressure in all cases was ambient.

Example 1 This example, which shows an attempt to descale carbon steelin a conventional hydride bath, is included only as a control.

Several 6" samples of carbon-steel rod (.225" gauge, hot rolled, 0.08%carbon) covered with oxide scale were dipped at 400 C. for fifteenminutes into molten sodium hydroxide containing 2% of sodium hydride andless than 0.1% iron. The rods were quenched in water while hot. Most ofthe reduced scale was found to adhere to the rods despite the quench. Itcould be removed only by acid pickling or by scrubbing or sprayingthoroughly with high pressure water.

Example 2 Through molten sodium hydroxide containing about 1% of iron(dissolved as an oxide of iron) and about 3% of sodium was bubbled amixture of nitrogen and hydrogen with a hydrogen partial pressure of0.02 to 0.1 atmosphere. formed and maintained in the bath. The sodiumoxide concentration was probably between /2% and 4% by weight based onconsideration of equilibrium constants.

Carbon-steel coupons covered with oxide scale were dipped into the bathat 400 C. for about five minutes, i

were withdrawn and then quenched in water without cooling. In each casethe reduced scale was completely or almost completely removed by thewater quench.

Example 3 3500 g. of sodium hydroxide were melted at 400 C. and dried bythe addition of a few grams of NazO. 50 g. of sodium was added, themixture was stirred, and 30 liters of hydrogen was passed thereover at arate of 1 liter/min. to generate sodium hydride in situ. The bath wasprotected from the atmosphere by a charcoal cover and a metal lid bothduring the generation of hydride and in subsequent operations. Afterabout 1% of hydride had been generated, a mixture of 100 g. of FezOs and100 g. of NazO was added. The bath was maintained non-corrosive to themetallic container or the workpieces at temperatures of 400, 450 and 500C. by the continuous passage of about 10 ml. of Hz/m-in. thereover.

This bath was operated over a period of five months without appreciablecorrosion of the tank or of samples About 2% of sodium hydride was thusat several different concentrations of hydride and iron. The hydrideconcentration was increased by adding sodium and stirring while passingover the bath 3 liters of Hz for every 5 g. of sodium. To increase theiron concentration, mixtures of sodium oxide and iron oxide (F6203) wereadded, usually in a weight ratio of 2:1. The iron concentration wasdecreased upon occasion by passing an excess of hydrogen over the batheither while hydride was being generated or while the bath was not inuse.

A strip of steel was weighed daily during the changes in conditions. Theconstancy of weight of the strip indicated that corrosion was negligiblewhen the variables were maintained within the ranges indicated above.Analyses for iron and hydride enables the bath to be controlled atwhatever set of conditions was desired.

Numerous steel samples of wire and rod having both high and low carboncontents were descaled in the hydride-oxide bath during the testingperiod. Reduced scale was removed completely or almost completely fromall samples by a simple water quench.

Having described my invention, I claim:

1. The process for removing oxide scale from the surface of an articleformed from a metal substantially nonreactive with alkali metalhydroxides which comprises contacting said article at a temperaturebelow the melting point thereof with a molten composition comprising amolten alkali metal hydroxide to which has been supplied about 03-10% byweight, based on weight of the total composition, of the oxide of analkali metal, about 0.3- 20% by weight of an alkali metal hydride, andabout 02-10% by weight of iron dissolved as one of its oxidic compoundsand subsequently removing the resulting reduced scale therefrom.

2. The process of claim 1 in which the alkali metal is sodium.

3. The process of claim 2 in which the temperature of the moltencomposition is about 325 -550 C.

4. The process of claim 2 in which the time of contact between thearticle and the composition is Az-30 minutes.

5. The process of claim 2 in which the article is made from a carbonsteel.

6. The process of claim 2 in which the resulting reduced scale isremoved by a water quench.

7. A fused bath for descaling ferrous articles carrying an oxide scalecomprising an alkali metal hydroxide to which has been supplied about03-10% by weight, based on the total bath weight, of the oxide of analkali metal, about 03-20% by weight of an alkali metal hydride, andabout 02-10% by weight of iron dissolved as one of its oxidic compounds.

8. The bath of claim 6 in which the alkali metal is sodium.

9. The method of forming a bath for descaling ferrous articles carryingan oxide scale which comprises dissolving about 02-10% by weight of ironin the form of one of its oxidic compounds in moltensodium hydroxide towhich has been supplied about 03-10% by weight, based on the total bathweight, of the oxide of an alkali metal, containing about 0.3-5% byweight of sodium hydride.

10. The method of claim 9 in which the iron is in the form of an oxideand is dissolved in the sodium hydroxide along with sodium oxide.

References Cited in the file of this patent UNITED STATES PATENTSGilbert Iuly 4, 1944 Gilbert June 12, 1945 OTHER REFERENCES Longmans,Green and Co., New York 1934.

1. THE PROCESS FOR REMOVING OXIDE SCALE FROM THE SURFACE OF AN ARTICLEFORMED FROM A METAL SUBSTANTIALLY NONREACTIVE WITH ALKALI METALHYDROXIDES WHICH COMPRISES CONTACING SAID ARTICLE AT A TEMPERATURE BELOWTHE MELTING POINT THEREOF WITH A MOLTEN COMPOSITION COMPRISING A MOLTENALKALI METAL HYDROXIDE TO WHICH HAS BEEN SUPPLIED ABOUT 0.3-10% BYWEIGHT, BASED ON WEIGHT OF THE TOTAL COMPOSITION, OF THE OXIDE OF ANALKALI METAL, ABOUT 0.320% BY WEIGHT OF AN ALKALI METAL HYDRIDE, ANDABOUT 0.2-10% BY WEIGHT OF IRON DISSOLVED AS ONE OF ITS OXIDIC COMPOUNDSAND SUBSEQUENTLY REMOVING THE RESULTING REDUCED SCALE THEREFROM.